The role of the uterus in regulation of heat duration in cycling rats

Estrogens synthesized and acting within a spinal oligomer suppress spinal endomorphin 2 antinociception: ebb and flow over the rat reproductive cycle

The magnitude of antinociception elicited by intrathecal endomorphin 2 (EM2), an endogenous mu-opioid receptor (MOR) ligand, varies across the rat estrous cycle. We now report that phasic changes in analgesic responsiveness to spinal EM2 result from plastic interactions within a novel membrane-bound oligomer containing estrogen receptors (mERs), aromatase (aka estrogen synthase), metabotropic glutamate receptor 1 (mGluR1), and MOR. During diestrus, spinal mERs, activated by locally synthesized estrogens, act with mGluR1 to suppress spinal EM2/MOR antinociception. The emergence of robust spinal EM2 antinociception during proestrus results from the loss of mER-mGluR1 suppression, a consequence of altered interactions within the oligomer. The chemical pairing of aromatase with mERs within the oligomer containing MOR and mGluR1 allows estrogens to function as intracellular messengers whose synthesis and actions are confined to the same signaling oligomer. This form of estrogenic signaling, which we term "oligocrine," enables discrete, highly compartmentalized estrogen/mER-mGluR1 signaling to regulate MOR-mediated antinociception induced by EM2. Finally, spinal neurons were observed not only to coexpress MOR, mERα, aromatase, and mGluR1 but also be apposed by EM2 varicosities. This suggests that modulation of spinal analgesic responsiveness to exogenous EM2 likely reflects changes in its endogenous analgesic activity. Analogous suppression of spinal EM2 antinociception in women (eg, around menses, comparable with diestrus in rats) as well as the (pathological) inability to transition out of that suppressed state at other menstrual cycle stages could underlie, at least in part, the much greater prevalence and severity of chronic pain in women than men.

Performance of Rodent Spermatozoa Over Time Is Enhanced by Increased ATP Concentrations: The Role of Sperm Competition

Sperm viability, acrosome integrity, motility, and swimming velocity are determinants of male fertility and exhibit an extreme degree of variation among closely related species. Many of these sperm parameters are associated with sperm ATP content, which has led to predictions of trade-offs between ATP content and sperm motility and velocity. Selective pressures imposed by sperm competition have been proposed as evolutionary causes of this pattern of diversity in sperm traits. Here, we examine variation in sperm viability, acrosome integrity, motility, swimming velocity, and ATP content over time, among 18 species of closely related muroid rodents, to address the following questions: (a) Do sperm from closely related species vary in ATP content after a period of incubation? (b) Are these differences in ATP levels related to differences in other sperm traits? (c) Are differences in ATP content and sperm performance over time explained by the levels of sperm competition in these species? Our results revealed a high degree of interspecific variability in changes in sperm ATP content, acrosome integrity, sperm motility and swimming velocity over time. Additionally, species with high sperm competition levels were able to maintain higher levels of sperm motility and faster sperm swimming velocity when they were incubated under conditions that support sperm survival. Furthermore, we show that the maintenance of such levels of sperm performance is correlated with the ability of sperm to sustain high concentrations of intracellular ATP over time. Thus, sperm competition may have an important role maximizing sperm metabolism and performance and, ultimately, the fertilizing capacity of spermatozoa.

Estrogens Suppress Spinal Endomorphin 2 Release in Female Rats in Phase with the Estrous Cycle

BACKGROUND/AIMS

Male and female rats differ in their ability to utilize spinal endomorphin 2 (EM2; the predominant mu-opioid receptor ligand in spinal cord) and in the mechanisms that underlie spinal EM2 analgesic responsiveness. We investigated the relevance of spinal estrogen receptors (ERs) to the in vivo regulation of spinal EM2 release.

METHODS

ER antagonists were administered directly to the lumbosacral spinal cord of male and female rats, intrathecal perfusate was collected, and resulting changes in EM2 release were quantified using a plate-based radioimmunoassay.

RESULTS

Intrathecal application of an antagonist of either estrogen receptor-α (ERα) or the ER GPR30 failed to alter spinal EM2 release. Strikingly, however, the concomitant blockade of ERα and GPR30 enhanced spinal EM2 release. This effect was sexually dimorphic, being absent in males. Furthermore, the magnitude of the enhancement of spinal EM2 release in females was dependent upon estrous cycle stage, suggesting a relationship with circulating levels of 17β-estradiol. The rapid onset of enhanced EM2 release following intrathecal application of ERα/GPR30 antagonists (within 30-40 min) suggests mediation via ERs in the plasma membrane, not the nucleus. Notably, both ovarian and spinally synthesized estrogens are essential for membrane ER regulation of spinal EM2 release.

CONCLUSION

These findings underscore the importance of estrogens for the regulation of spinal EM2 activity and, by extension, endogenous spinal EM2 antinociception in females. Components of the spinal estrogenic mechanism(s) that suppress EM2 release could represent novel drug targets for improving utilization of endogenous spinal EM2, and thereby pain management in women.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Importance of sex to pain and its amelioration; relevance of spinal estrogens and its membrane receptors

Estrogens have a multitude of effects on opioid systems and are thought to play a key role in sexually dimorphic nociception and opioid antinociception. Heretofore, classical genomic actions of estrogens are largely thought to be responsible for the effects of these steroids on nociception and opioid antinociception. The recent discovery that estrogens can also activate estrogen receptors that are located in the plasma membrane, the effects of which are manifest in seconds to minutes instead of hours to days has revolutionized our thinking concerning the ways in which estrogens are likely to modulate pain responsiveness and the dynamic nature of that modulation. This review summarizes parameters of opioid functionality and nociception that are subject to modulation by estrogens, underscoring the added dimensions of such modulation that accrues from rapid membrane estrogen receptor signaling. Implications of this mode of signaling regarding putative sources of estrogens and its degradation are also discussed.

Males show stronger contextual fear conditioning than females after context pre-exposure

Estradiol affects the structure and function of the hippocampus. We have found that repeated estradiol affects neurogenesis and cell death in the hippocampus of adult female, but not male rats. In the present study we sought to determine whether using the same regimen of estradiol would influence hippocampus-dependent behaviour. Adult male and female rats were given estradiol or sesame oil for 15 days, and then tested using a contextual pre-exposure paradigm in which performance depends on the hippocampus. The time spent freezing displayed by rats was scored on subsequent days in (1) the training context, (2) a novel context in which rats had never been shocked, and (3) the training context a second time. Irrespective of treatment, males showed stronger memory for the context by exhibiting greater freezing in both the training context exposures and the novel context. Previous estradiol treatment, in either sex, did not affect the ability to learn and retain information about the training context. However, female rats treated with estradiol and exposed to a novel context after fear conditioning exhibited less freezing behaviour than controls. Taken together, our results demonstrate that gonadectomized male rats outperform females, regardless of previous treatment with estradiol, on a hippocampus-contextual fear conditioning test, and that previous estradiol treatment has a subtle effect on performance in female but not male rats.

Functional significance of the rapid regulation of brain estrogen action: where do the estrogens come from?

Estrogens exert a wide variety of actions on reproductive and non-reproductive functions. These effects are mediated by slow and long lasting genomic as well as rapid and transient non-genomic mechanisms. Besides the host of studies demonstrating the role of genomic actions at the physiological and behavioral level, mounting evidence highlights the functional significance of non-genomic effects. However, the source of the rapid changes in estrogen availability that are necessary to sustain their fast actions is rarely questioned. For example, the rise of plasma estrogens at pro-estrus that represents one of the fastest documented changes in plasma estrogen concentration appears too slow to explain these actions. Alternatively, estrogen can be synthesized in the brain by the enzyme aromatase providing a source of locally high concentrations of the steroid. Furthermore, recent studies demonstrate that brain aromatase can be rapidly modulated by afferent inputs, including glutamatergic afferents. A role for rapid changes in estrogen production in the central nervous system is supported by experiments showing that acute aromatase inhibition affects nociception as well as male sexual behavior and that preoptic aromatase activity is rapidly (within min) modulated following mating. Such mechanisms thus fulfill the gap existing between the fast actions of estrogen and their mode of production and open new avenues for the understanding of estrogenic effects on the brain.

Food deprivation inhibits estrous behavior in hormone-treated Syrian hamsters despite elevated estradiol levels

Estradiol and progesterone (P) induce female mammalian reproductive behaviors, which are, in turn, sensitive to food availability. When ovariectomized, steroid-primed hamsters are food deprived for 48 h, estrous behavior is suppressed. While this suppression of estrous behavior may be due to alterations in neural steroid receptor levels, it is also possible that decreased levels of circulating estradiol could be involved in mediating this suppression. Ovariectomized Syrian hamsters given varying doses of estradiol benzoate (EB) and P were tested to determine whether increasing doses of sex steroids would overcome the suppressive effects of food deprivation on estrous behavior. As expected, lordosis duration decreased in food-deprived animals. Increasing the levels of EB, but not P, increased lordosis duration in the food-deprived animals so that animals who were given 20 microg of EB had lordosis durations significantly longer than food-deprived hamsters that received 1.5 microg and 2.5 microg EB. Following an injection of 2.5 microg of EB, food-deprived hamsters actually had higher circulating levels of estradiol than ad libitum-fed animals. Therefore, increasing circulating levels of estradiol can increase lordosis durations in fasted animals; however, the suppression of estrous behavior occurs despite increased circulating estradiol levels in ovariectomized, steroid-treated animals. The most parsimonious explanation for this phenomenon is a deprivation-induced reduction in neural responsiveness to estradiol.

Copulatory stimuli in rats induce heat abbreviation through effects on genitalia but not through effects on central nervous mechanisms supporting the steroid hormone-induced sexual responsiveness

Male copulatory stimuli in various animal species abbreviate the length of the period of their female conspecifics' heat. Such effect can be explained in some species as the result of copulation-induced alterations in ovarian hormone secretion (e.g. reflex ovulators). Other species require a different explanation: specifically, interactions in the hypothalamus between effects of steroid hormones and inhibitory neural afferents from the genital area have been hypothesized to play a role in some laboratory animals such as guinea pigs, hamsters and rats. However, the evidence in support of heat abbreviation through copulatory stimuli in rats is at best equivocal. There is obvious need for its unequivocal demonstration prior to any further analysis of the potential mechanism of action. The present study intended to find a model in support of the concept of 'centrally induced' heat abbreviation in rats. Virgin rats during 'natural heat' were examined with or without allowing them the opportunity to pace male copulatory activities. Ovariectomized, steroid hormone-treated rats were studied with minor or ample experience with sexual activities in prior tests and, also, with or without the opportunity to 'pace' male sexual acts. None of the experimental models revealed unequivocal support for detrimental effect of large numbers of intromittive copulations on 'central mechanisms' regulating heat duration. It rather seemed that frequent intromittive copulations and ejaculates affected the 'peripheral' genital tract (vulva, vagina, cervix) making further copulation of a highly aversive quality so that sexual encounters were avoided or prevented. This effect was noticed with great inter-individual variability. The conclusion is drawn that rats do not show the counterpart of heat abbreviation reportedly occurring rapidly and reliably in guinea pigs after a limited amount of vaginal/cervical stimulation through copulation or insertions of a glass-rod.

Interactions between oestradiol and the progesterone antagonist RU-486 in establishing and maintaining female rats' sexual responsiveness: central versus peripheral effects

Progesterone is well known to contribute specifically to the emergence of the female rats' sexual behaviour by the establishment of 'proceptivity'. Analysis of the mechanism of progesterone action benefits from the availability of highly effective anti-progestagenic compounds. However, results obtained during the study of female rats' sexual behaviour, including such compounds into the experimental protocol, appear equivocal. The present experiments were designed to further examine the possible effects of the antiprogestagenic compound RU-486 (Mifepristone) on the female rats' sexual responsiveness as elicited through exposure of the animals to oestradiol alone. The experimental design aimed to distinguish between receptivity (defined as response to sexually active males) and proceptivity (defined as female-initiated sexual behaviour). Mifepristone advanced the onset of receptivity after the injection of oestradiol benzoate (OB). Upon further investigation a steady level of receptivity was reached during prolonged treatment with OB and this level appeared unaltered through concurrent treatment with Mifepristone. OB alone was insufficient to induce full proceptivity as revealed by observations of sexual behaviour with tethered males. Such defined proceptivity was significantly further inhibited by Mifepristone. It thus appears that, dependent upon the time and type of female sexual behavioural analysis, Mifepristone either enhances, inhibits, or does not affect sexual responsiveness. After the observation period, autopsy revealed the presence of copulatory plugs and infections in the uterus of OB + Mifepristone-treated rats. This unexpected finding could result from effects of the compound on the uterine cervical musculature. Uterine infections might result in painful, aversive, intra-abdominal sensations, especially during intravaginal penile intromissions.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothalamic nuclear progestin receptors and the duration of sexual receptivity in ovariectomized and ovariectomized-hysterectomized rats

In order to determine if the enhancement of sexual behavior in hysterectomized rats is associated with an increased level of hypothalamic nuclear progestin receptors, ovariectomized (OVX) and ovariectomized-hysterectomized (OVHX) rats were injected with 2 micrograms estradiol benzoate. Twenty-four hr later, animals were injected with 0.5 mg progesterone and were tested for sexual receptivity every 4 hr. Hysterectomy had an overall facilitatory effect on lordosis and increased the duration of the period of sexual receptivity by about 4 hr. In a second experiment, similarly-treated animals were killed 4, 8, or 12 hr after progesterone injection, and hypothalamic nuclear progestin receptor levels were measured. In contrast to what has been reported for guinea pigs, nuclear progestin receptor levels decreased to baseline 8-12 hr before the termination of sexual receptivity. Nuclear progestin receptor concentrations were higher in OVHX rats than in OVX rats at 4 hr after progesterone injection, and there was a trend toward higher receptor concentrations in OVHX rats at 8 hr. These results demonstrate that hysterectomy-induced facilitation of sexual receptivity is associated with an increased level of hypothalamic nuclear progestin receptors. Furthermore, they suggest a fundamental difference in the regulation of nuclear progestin receptor retention between rats and guinea pigs.